1. Field of the Invention
The invention relates to sol-gel processing methods.
2. Discussion of the Related Art
Optical fiber is produced from a glass preform, the preform typically consisting of a doped silica core surrounded by one or more claddings. As discussed in F. DiMarcello et al. "Fiber Drawing and Strength Properties," Optical Fiber Communications, Vol. 1, Academic Press, Inc., 1995, at 179-248, the disclosure of which is hereby incorporated by reference, the preform is generally arranged vertically in a draw tower such that a portion of the preform is lowered into a furnace region that typically heats the preform to temperatures around 2200.degree. C. The portion of the preform placed into the furnace region begins to melt, and the lower end of the preform forms what is known as the neck-down region, which is where the preform glass flows from the original cross-sectional area of the preform to the desired cross-sectional area of the fiber. From the lower tip of this neck-down region, the optical fiber is drawn.
One approach to preform manufacture involves the fabrication of an overcladding that surrounds an inner cladding and core. The overcladding does not have to meet specifications as precise as the core and inner cladding, and efforts to speed manufacture of preforms have therefore often focused on less expensive methods of forming the overcladding. One manner of forming the overcladding is the use of a sol-gel process. However, sol-gel methods have in the past tended to encounter cracking during the overcladding tube formation and subsequent drying process. Methods that suppressed such tendency included, for example, the use of supercritical drying and/or the use of drying control chemical additives (DCCA), both of which are relatively expensive and laborious. Other sol-gel processes have involved the precipitation of silica particles from solution. However, such precipitation processes typically involved the use of alkali silicates, and thus required further processing steps to remove the alkali metal ions.
U.S. Pat. No. 5,240,488, the disclosure of which is hereby incorporated by reference, discloses a sol-gel process capable of producing crack-free overcladding preform tubes of a kilogram or larger. In this process, a colloidal silica dispersion, e.g., fumed silica, is obtained having a pH of 2 to 4. To obtain adequate stability of the dispersion and prevent agglomeration, the pH is raised to a value of about 10 to about 14 by use of a base. Typically, a commercially-obtained dispersion is pre-stabilized at such a pH value by addition of a base such as tetramethylammonium hydroxide (TMAH). Introduction of the TMAH raises the pH value. Other quaternary ammonium hydroxides behave similarly. When the pH is so raised, the silica takes on a negative surface charge due to ionization of silanol groups present on the surface, in accordance with the following reaction: EQU --Si--OH+OH.sup.- {character pullout}--Si--O.sup.- +H.sub.2 O.
The negative charge of the silica particles creates mutual repulsion, preventing substantial agglomeration and maintaining the stability of the dispersion. In this state, the zeta potential of the particles is at a negative value. (Zeta potential is the potential across the diffuse layer of ions surrounding a charged colloidal particle, and is typically measured from electrophoretic mobilities--the rate at which colloidal particles travel between charged electrodes placed in a solution. See, e.g., C. J. Brinker and G. W. Scherer, Sol-Gel Science, Academic Press, 242-243.)
At a later stage in the process, as discussed in Col. 15, lines 39-65 of the '488 patent, a gelling agent such as methyl formate is added to reduce the pH. It is possible to use other esters, as well. The ester reacts to neutralize base, and the negative character of the silica particles is neutralized according to the following reaction: EQU --Si--O.sup.- +H+{character pullout}--Si--OH.
A sufficient amount of the ester must be introduced to neutralize the silica to a degree where gelation is induced. (Gelation, as used herein, indicates that the colloidal silica particles have formed a three-dimensional network with some interstitial liquid, such that the dispersion becomes essentially non-flowing, e.g., exhibiting solid-like behavior, at room temperature.)
Subsequent to gelation, the sol-gel body is typically released from its mold, and placed in an oven for drying and subsequent heat treatment, as reflected in the Table at Cols. 11-12 of the '488 patent. The gelled body is relatively weak and brittle, e.g., typically having an ultimate strength of no more than 0.5 MPa when released from a mold (ultimate strength measured by a conventional 3- or 4-point bending test). Because of the relative weakness of the sol-gel body, internal stresses induced by drying are capable of causing cracks in the body, as discussed in G. Scherer, "Stress and Fracture During Drying of Gels," Journal of Non-Crystalline Solids, Vol. 121, 1990, 104. To reduce the likelihood of cracking, sol-gel bodies are typically dried slowly, e.g., from several days to about two weeks at relatively low temperatures of 10 to 30.degree. C. and relatively high humidity of at least 50%. Drying thus constitutes a substantial portion of the overall production time of a sol-gel body, often creating a bottle-neck in a production line. Moreover, the body must be handled gently upon release from the mold to prevent deformation and breakage, thereby complicating the overall fabrication process.
Improved sol-gel processes are therefore desired, in which stronger sol-gel bodies are formed and/or shorter drying times are possible.